Broke down API to be even more modular. Further shortened error methods.

- Error methods were still akward to type. Made them even shorter.
- Inlined the logic around tappability to replace public `isTappable`
with `isProbablyTappable`.
- Removed some unused helpers that were creating clutter.
- Broke
`+accessibilityElement:view:withLabel:value:traits:tappable:error to be
two methods so now you can comforably swap out the existing logic in a
few lines of code.
- Fully documented the UIAccessibilityElement additions since they are
so central to compositing your own tests.
Added additional docs for UIApplication.
Small test cleanup.  Added docs on test action composition.
Added special behavior for scrolling to the end of table views.

> For cases where you may need to work from the end of a table view
rather than the beginning, negative sections count back from the end of
the table view (-1 is the last section) and negative rows count back
from the end of the section (-1 is the last row for that section).
Bumped the version to 2.0.0pre4